This invention relates to coated particles and more particularly to particles having a conformal coating thereon. More particularly, this invention relates to phosphors and still more particularly to electroluminescent phosphors having thereon a coating that protects the phosphor from moisture absorption and greatly increases the life and efficacy.
Coated phosphors are known from U.S. Pat. Nos. 4,585,673; 4,825,124; 5,080,928; 5,118,529; 5,156,885; 5,220,243; 5,244,750; and 5,418,062. It is known from some of the just-mentioned patents that a coating precursor and oxygen can be used to apply a protective coating. See, for example, U.S. Pat. Nos. 5,244,750 and 4,585,673. The coating processes in several of the others of these patents employ chemical vapor deposition to apply a protective coating by hydrolysis. Additionally, the above-cited U.S. patent application Ser. No.: 09/153,978, filed Sep. 16, 1998, now abandoned, discloses a method for coating phosphor particles by chemical vapor deposition and using an oxygen/ozone reactant. The latter process operates in the absence of water or water vapor. The above-cited U.S. patent application Ser. No. 09/177,226, filed Oct. 22, 1998, now abandoned, discloses an improvement to the oxygen/ozone process which further increases the life and efficacy by first saturating the phosphor with a precursor before beginning the deposition. It would be a still further advance in the art to increase the efficacy and the life of such coated phosphors even more.
It is, therefore, an object of the invention to obviate the disadvantages of the prior art.
It is another object of the invention to enhance the operation of coated phosphors.
Yet another object of the invention is the provision of a phosphor coating method that does not employ water or water vapor.
These objects are accomplished, in one aspect of the invention, by a method of coating phosphor particles with the steps comprising: introducing an inert gas into a reaction vessel; charging phosphor particles into said reaction vessel; heating said reaction vessel to a reaction temperature; introducing a coating precursor which includes carbon into said reaction vessel for a time sufficient to saturate said phosphor particles with said precursor; continuing precursor flow into said reaction vessel; introducing an oxygen/ozone mixture into said reaction vessel, said oxygen/ozone mixture comprising less than 4.4 wt. % ozone; and maintaining said inert gas flow, oxygen/ozone mixture flow and further precursor supply for a time sufficient to coat said phosphor particles.
It has been found that, when during the above-cited process, the ozone generator is operated at far less than maximum efficiency and the coating precursor contains carbon, a phosphor particle will be produced having a coating which contains from about 2200 to about 6300 ppm carbon. These phosphor particles, when used to manufacture electroluminescent lamps, provide lamps having efficacies in the range of 6.1 to 7.7 lumens/watt, far in excess of the 3.3 lumens/watt of the control.
For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims.
Referring now to the invention with greater particularity, there is provided a method for preparing high-efficacy and long-life electroluminescent (EL) phosphors by coating such phosphors from an organic, metal containing precursor such as trimethyl aluminum (TMA). The method employs chemical vapor deposition to deposit the coating on individual phosphor particles and particularly concerns coating the phosphors in a manner to include in the coating substantial amounts of carbon.
An inert nitrogen gas stream, which bypasses the TMA bubbler, (5.0 liter/minute flow rate) was first flowed into the bottom of an empty, 4 inch diameter quartz tube which is used as the fluidized bed reactor. 10.0 kg of copper-doped zinc sulfide (ZnS:Cu) electroluminescent phosphor, such as Sylvania Type 728, available from Osram Sylvania Products Inc., Towanda, Pa., was charged in the quartz tube, which has a length of 36 inches. The phosphor particles were suspended by the stream of nitrogen gas in the fluidized bed reactor with a bed height of about 18 inches. A vibromixer was turned on and operated at a speed of 60 cycles /minute and the bed was heated to a temperature of approximately 180xc2x0 C. by an external furnace. A thermocouple positioned at the middle of the powder be was used to control the reactor temperature within xc2x13xc2x0 C. during the coating process. When the temperature approaches 180xc2x0 C., a TMA pretreatment step was initiated with nitrogen gas flowing through the TMA bubbler at 2.0 liters/minute. The TMA bubbler is kept at a temperature of 34xc2x0 C. and maintains a constant TMA vapor pressure. The second nitrogen gas stream containing the vaporized TMA coating precursor was mixed with the 5.0 liters/minute nitrogen gas stream that bypassed the TMA bubbler and flowed into the base of the fluidized bed reactor. This dilute TMA precursor vapor passed through a metal frit located under the tube reactor and used to support the phosphor particle bed. After the surfaces of the phosphor particles were saturated with the TMA precursor for 10 minutes, oxygen gas with a flow rate of 16.5 liter/minute was passed through a Model GL-1 ozone generator manufactured by PCI Ozone and Control Systems, Inc. The ozone production rate of the generator was controlled by varying the DC current to the inverter. With various ozone output rate settings, different amounts of ozone gas were produced from the ozone generator. The oxygen/ozone mixture was flowed into the reactor through a series of holes circumferentially located on the hollow shaft of the vibromixer, above the vibrating disc, to start the coating process. The final product was collected at the end of the coating experiment (about 70 hours) for chemical analyses and lamp evaluation.